Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball showing a low spin rate on driver shots and an excellent shot feeing. The present invention provides a golf ball having a spherical core and at least one cover covering the spherical core, wherein the spherical core is formed from a rubber composition containing: (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and at least one compound of (d) mercaptobenzoic acids and/or salts thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, and the spherical core has a hardness difference (Hs—Ho) between a surface hardness (Hs) and a center hardness (Ho) thereof of 20 or more in JIS-C hardness.

FIELD OF THE INVENTION

The present invention relates to a golf ball showing a low spin rate on driver shots, in particular, an improvement of a core of a golf ball.

DESCRIPTION OF THE RELATED ART

As a method for improving a flight distance on driver shots, for example, there are methods of using a core having high resilience and using a core having a hardness distribution in which the hardness increases toward the surface of the core from the center thereof. The former method has an effect of enhancing an initial speed, and the latter method has an effect of a higher launch angle and a lower spin rate. A golf ball having a higher launch angle and a low spin rate travels a great distance.

For example, Japanese Patent Publications Nos. S61-37178 A, S61-113475 A, S61-253079 A, 2008-212681 A, 2008-523952 T and 2009-119256 A disclose a technique of enhancing resilience of the core. Japanese Patent Publications Nos. S61-37178 A and S61-113475 A disclose a solid golf ball having an inner core where zinc acrylate as a co-crosslinking agent, palmitic acid, stearic acid, or myristic acid as a co-crosslinking activator, zinc oxide as another co-crosslinking activator, and a reaction rate retarder are blended, with respect to 100 parts by weight of a rubber.

Japanese Patent Publication No. S61-253079 A discloses a solid golf ball formed from a rubber composition containing an α,β-unsaturated carboxylic acid in an amount of 15 parts to 35 parts by weight, a metal compound to react with the α,β-unsaturated carboxylic acid and form a salt thereof in an amount of 7 parts to 60 parts by weight, and a high fatty acid metal salt in an amount of 1 part to 10 parts by weight with respect to 100 parts by weight of a base rubber.

Japanese Patent Publication No. 2008-212681 A discloses a golf ball comprising, as a component, a molded and crosslinked product obtained from a rubber composition essentially comprising a base rubber, a filler, an organic peroxide, an α,β-unsaturated carboxylic acid and/or a metal salt thereof, a copper salt of a saturated or unsaturated fatty acid.

Japanese Patent Publication No. 2008-523952 T discloses a golf ball, or a component thereof, molded from a composition comprising a base elastomer selected from the group consisting of polybutadiene and mixtures of polybutadiene with other elastomers, at least one metallic salt of an unsaturated monocarboxylic acid, a free radical initiator, and a non-conjugated diene monomer.

Japanese Patent Publication No. 2009-119256 A discloses a method of manufacturing a golf ball, comprising preparing a masterbatch of an unsaturated carboxylic acid and/or a metal salt thereof by mixing the unsaturated carboxylic acid and/or the metal salt thereof with a rubber material ahead, using the masterbatch to prepare a rubber composition containing the rubber material, and employing a heated and molded product of the rubber composition as a golf ball component, wherein the masterbatch of the unsaturated carboxylic acid and/or the metal salt thereof comprises: (A) from 20 wt % to 100 wt % of a modified polybutadiene obtained by modifying a polybutadiene having a vinyl content of from 0 to 2%, a cis-1,4 bond content of at least 80% and active terminals, the active terminal being modified with at least one type of alkoxysilane compound, (B) from 80 wt % to 0 wt % of a diene rubber other than (A) the above rubber component [the figures are represented by wt % in the case that a total amount of (A) and (B) equal to 100 wt %] and (C) an unsaturated carboxylic acid and/or a metal salt thereof.

For example, Japanese Patent Publications Nos. H6-154357 A, 2008-194471 A, 2008-194473 A and 2010-253268 A disclose a core having a hardness distribution. Japanese Patent Publication No. H6-154357 A discloses a two-piece golf ball comprising a core formed of a rubber composition containing a base rubber, a co-crosslinking agent, and an organic peroxide, and a cover covering said core, wherein the core has the following hardness distribution according to JIS-C type hardness meter readings: (1) hardness at center: 58-73, (2) hardness at 5 to 10 mm from center: 65-75, (3) hardness at 15 mm from center: 74-82, (4) surface hardness: 76-84, wherein hardness (2) is almost constant within the above range, and the relation (1)<(2)<(3)≦(4) is satisfied.

Japanese Patent Publication No. 2008-194471 A discloses a solid golf ball comprising a solid core and a cover layer that encases the core, wherein the solid core is formed of a rubber composition composed of 100 parts by weight of a base rubber that includes from 60 to 100 parts by weight of a polybutadiene rubber having a cis-1,4 bond content of at least 60% and synthesized using a rare-earth catalyst, from 0.1 to 5 parts by weight of an organic sulfur compound, an unsaturated carboxylic acid or a metal salt thereof, an inorganic filler, and an antioxidant; the solid core has a deformation amount from 2.0 mm to 4.0 mm, when applying a load from an initial load of 10 kgf to a final load of 130 kgf and has the hardness distribution shown in the following table.

TABLE 1 Hardness distribution in solid core Shore D harness Center 30 to 48 Region located 4 mm from center 34 to 52 Region located 8 mm from center 40 to 58 Region located 12 mm from center (Q) 43 to 61 Region located 2 to 3 mm inside of surface (R) 36 to 54 Surface (S) 41 to 59 Hardness difference [(Q)-(S)]  1 to 10 Hardness difference [(S)-(R)]  3 to 10

Japanese Patent Publication No. 2008-194473 A discloses a solid golf ball comprising a solid core and a cover layer that encases the core, wherein the solid core is formed of a rubber composition composed of 100 parts by weight of a base rubber that includes from 60 to 100 parts by weight of a polybutadiene rubber having a cis-1,4 bond content of at least 60% and synthesized using a rare-earth catalyst, from 0.1 to 5 parts by weight of an organic sulfur compound, an unsaturated carboxylic acid or a metal salt thereof, and an inorganic filler; the solid core has a deformation amount from 2.0 mm to 4.0 mm, when applying a load from an initial load of 10 kgf to a final load of 130 kgf and has the hardness distribution shown in the following table.

TABLE 2 Hardness distribution in solid core Shore D harness Center 25 to 45 Region located 5 to 10 mm from center 39 to 58 Region located 15 mm from center 36 to 55 Surface (S) 55 to 75 Hardness difference between center and surface 20 to 50

Japanese Patent Publication No. 2010-253268 A discloses a multi-piece solid golf ball comprising a core, an envelope layer encasing the core, an intermediate layer encasing the envelope layer, and a cover which encases the intermediate layer and has formed on a surface thereof a plurality of dimples, wherein the core is formed primarily of a rubber material and has a hardness which gradually increases from a center to a surface thereof, the hardness difference in JIS-C hardness units between the core center and the core surface being at least 15 and, letting (I) be the average value for cross-sectional hardness at a position about 15 mm from the core center and at the core center and letting (II) be the cross-sectional hardness at a position about 7.5 mm from the core center, the hardness difference (I)-(II) in JIS-C units being within ±2; and the envelope layer, intermediate layer and cover have hardness which satisfy the condition: cover hardness>intermediate layer hardness>envelope layer hardness.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a golf ball showing a low spin rate on driver shots and an excellent shot feeling.

The present invention provides a golf ball having a spherical core and at least one cover covering the spherical core, wherein the spherical core is formed from a rubber composition containing: (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and at least one compound of (d) mercaptobenzoic acids and/or salts thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, and the spherical core has a hardness difference (Hs—Ho) between a surface hardness (Hs) and a center hardness (Ho) thereof of 20 or more in JIS-C hardness.

The golf ball of the present invention has a spherical core having a higher degree of the outer-hard inner-soft structure without increasing the surface hardness thereof excessively, by blending (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof and at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof in the rubber composition used for forming the spherical core. Therefore, the golf ball of the present invention has a low spin rate on driver shots and an excellent shot feeling.

The action provided by at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof in the rubber composition used for the golf ball of the present invention is considered as follows. The metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms blended in the rubber composition is considered to form an ion cluster in the core, thereby crosslinking the rubber molecules with metals. By blending at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof into this rubber composition, at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof exchanges a cation with the ion cluster formed from the metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, thereby breaking the metal crosslinking by the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. This cation exchange reaction easily occurs at the core central part where the temperature is high, and less occurs toward the core surface. When molding a core, the internal temperature of the core is high at the core central part and decreases toward the core surface, since reaction heat from a crosslinking reaction of the base rubber accumulates at the core central part. In other words, the breaking of the metal crosslinking by at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof easily occurs at the core central part, but less occurs toward the surface. As a result, it is conceivable that since a crosslinking density in the core increases from the center of the core toward the surface thereof, the core hardness increases from the center of the core toward the surface thereof.

The present invention provides a golf ball showing a low spin rate on driver shots and an excellent shot feeling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway sectional view showing the golf ball according to the preferable embodiment of the present invention;

FIG. 2 is a graph showing the hardness distribution of the spherical core;

FIG. 3 is a graph showing the hardness distribution of the spherical core;

FIG. 4 is a graph showing the hardness distribution of the spherical core;

FIG. 5 is a graph showing the hardness distribution of the spherical core;

FIG. 6 is a graph showing the hardness distribution of the spherical core;

FIG. 7 is a graph showing the hardness distribution of the spherical core;

FIG. 8 is a graph showing the hardness distribution of the spherical core;

FIG. 9 is a graph showing the hardness distribution of the spherical core;

FIG. 10 is a graph showing the hardness distribution of the spherical core; and

FIG. 11 is a graph showing the hardness distribution of the spherical core.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The golf ball of the present invention has a spherical core and at least one cover covering the spherical core, wherein the spherical core is formed from a rubber composition containing: (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and at least one compound of (d) mercaptobenzoic acids and/or salts thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

First, (a) the base rubber used in the present invention will be explained. As (a) the base rubber used in the present invention, natural rubber and/or synthetic rubber can be used. For example, polybutadiene rubber, natural rubber, polyisoprene rubber, styrene polybutadiene rubber, ethylene-propylene-diene rubber (EPDM), or the like can be used. These rubbers may be used solely or two or more of these rubbers may be used in combination. Among them, typically preferred is the high cis-polybutadiene having a cis-1,4 bond in a proportion of 40% or more, more preferably 80% or more, even more preferably 90% or more in view of its superior resilience property.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in a content of 2 mass % or less, more preferably 1.7 mass % or less, and even more preferably 1.5 mass % or less. If the content of 1,2-vinyl bond is excessively high, the resilience may be lowered.

The high-cis polybutadiene preferably includes one synthesized using a rare earth element catalyst. When a neodymium catalyst, which employs a neodymium compound of a lanthanum series rare earth element compound, is used, a polybutadiene rubber having a high content of a cis-1,4 bond and a low content of a 1,2-vinyl bond is obtained with excellent polymerization activity. Such a polybutadiene rubber is particularly preferred.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 or more, more preferably 32 or more, even more preferably 35 or more, and preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 140 or less, more preferably 120 or less, even more preferably 100 or less, and most preferably 80 or less. It is noted that the Mooney viscosity (ML₁₊₄(100° C.)) in the present invention is a value measured according to JISK6300 using an L rotor under the conditions of: a preheating time of 1 minute; a rotor revolution time of 4 minutes; and a temperature of 100° C.

The high-cis polybutadiene preferably has a molecular weight distribution Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) of 2.0 or more, more preferably 2.2 or more, even more preferably 2.4 or more, and most preferably 2.6 or more, and preferably has a molecular weight distribution Mw/Mn of 6.0 or less, more preferably 5.0 or less, even more preferably 4.0 or less, and most preferably 3.4 or less. If the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is excessively low, the processability may deteriorate. If the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is excessively high, the resilience may be lowered. It is noted that the measurement of the molecular weight distribution is conducted by gel permeation chromatography (“HLC-8120GPC”, manufactured by Tosoh Corporation) using a differential refractometer as a detector under the conditions of column: GMHHXL (manufactured by Tosoh Corporation), column temperature: 40° C., and mobile phase: tetrahydrofuran, and calculated by converting based on polystyrene standard.

Next, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof will be explained. (b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is blended as a co-crosslinking agent in the rubber composition and has an action of crosslinking a rubber molecule by graft polymerization to a base rubber molecular chain. In the case that the rubber composition used in the present invention contains only the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, the rubber composition preferably further contains (e) a metal compound as an essential component. Neutralizing the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with the metal compound in the rubber composition provides substantially the same effect as using the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Further, in the case of using the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and the metal salt thereof in combination, (e) the metal compound may be used as an optional component.

The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includes, for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid, and the like.

Examples of the metals constituting the metal salts of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include: monovalent metal ions such as sodium, potassium, lithium or the like; divalent metal ions such as magnesium, calcium, zinc, barium, cadmium or the like; trivalent metal ions such as aluminum ion or the like; and other metal ions such as tin, zirconium or the like. The above metal ions can be used solely or as a mixture of at least two of them. Among these metal ions, divalent metal ions such as magnesium, calcium, zinc, barium, cadmium or the like are preferable. Use of the divalent metal salts of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms easily generates a metal crosslinking between the rubber molecules. Especially, as the divalent metal salt, zinc acrylate is preferable, because zinc acrylate enhances the resilience of the resultant golf ball. The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof may be used solely or in combination of at least two of them.

The content of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is preferably 15 parts by mass or more, more preferably 20 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, even more preferably 35 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the content of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is less than 15 parts by mass, the content of (c) the crosslinking initiator which will be explained below must be increased in order to obtain the appropriate hardness of the constituting member formed from the rubber composition, which tends to cause the lower resilience. On the other hand, if the content of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof exceeds 50 parts by mass, the constituting member formed from the rubber composition becomes excessively hard, which tends to cause the lower shot feeling.

(c) The crosslinking initiator is blended in order to crosslink (a) the base rubber component. As (c) the crosslinking initiator, an organic peroxide is preferred. Specific examples of the organic peroxide include organic peroxides such as dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. These organic peroxides may be used solely or two or more of these organic peroxides may be used in combination. Among them, dicumyl peroxide is preferably used.

The content of (c) the crosslinking initiator is preferably 0.2 part by mass or more, and more preferably 0.5 part by mass or more, and is preferably 5.0 parts by mass or less, and more preferably 2.5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the content of (c) the crosslinking initiator is less than 0.2 part by mass, the constituting member formed from the rubber composition becomes so soft that the golf ball may have the lower resilience. If the content of (c) the crosslinking initiator exceeds 5.0 parts by mass, the amount of (b) the co-crosslinking agent must be decreased in order to obtain the appropriate hardness of the constituting member formed from the rubber composition, resulting in the insufficient resilience and lower durability of the golf ball.

The rubber composition used for the present invention contains at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof. In the present invention, it is important to make a reaction between at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof and (b) the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent. This reaction controls the degree of the crosslinking introduced by the co-crosslinking agent, thereby enhancing the degree of the outer-hard inner-soft structure of the core. Furthermore, by blending at least one compound of (d) the mercaptobenzoic acids and/or the salts, the core shows a higher degree of the outer-hard inner-soft structure without increasing the surface hardness thereof excessively.

(d) The mercaptobenzoic acids preferably include mercaptobenzoic acid and/or a derivative thereof. Mercaptobenzoic acid and/or the derivative thereof includes a compound represented by the following formula (1).

(in formula (1), each R independently represents halogen, a hydroxyl group, an alkyl group, an aryl group, an aralkyl group, an alkylaryl group, an alkoxyl group, an amino group which may be substituted, a cyano group, an oxo group, a carboxyl group, a thiocarboxyl group and a mercapto group; n represents an integer number of 0 to 4; when n is 2 or more, each R may be same or different.)

The halogen includes, for example, fluorine, chlorine, bromine, and iodine.

The alkyl group includes a methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, and decyl group. Among them, preferred is an alkyl group having 1 to 6 carbon atoms, and more preferred is an alkyl group having 1 to 4 carbon atoms. The aryl group includes a phenyl group, naphthyl group, anthryl group, biphenyl group, phenanthryl group, and fluorenyl group. Among them, preferred is a phenyl group. The aralkyl group includes a benzyl group, phenethyl group, phenylpropyl group, naphthylmethyl group, and naphthylethyl group. The alkylaryl group includes a tolyl group, xylyl group, cumenyl group, and mesityl group.

The alkoxyl group includes a methoxy group, ethoxy group, propoxy group, butoxy group, pentyloxy group, hexyloxy group, heptyloxy group, octyloxy group, nonyloxy group, and decyloxy group. Among them, preferred is an alkoxyl group having 1 to 6 carbon atoms, and more preferred is an alkoxyl group having 1 to 4 carbon atoms.

The amino group which may be substituted preferably includes an amino group where one or more hydrogen atoms in the amino group are substituted with an alkyl group or aryl group. The amino group which may be substituted includes a methylamino group, dimethyl amino group, ethylamino group, propylamino group, isopropylamino group, butylamino group, isobutylamino group, tert-butylamino group, pentylamino group, hexylamino group, 2-ethylhexylamino group, phenylamino group, diphenylamino group, and naphthylamino group.

(d) The salts of the mercaptobenzoic acids includes a salt of the compound represented by the formula (1) (mercaptobenzoic acid and/or the derivative thereof), and the like. The cation component of the salt may be any one of a metal ion, an ammonium ion and an organic cation. The metal ion includes, for example, monovalent metal ions such as sodium, potassium, lithium, silver, and the like; bivalent metal ions such as magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel, manganese, and the like; trivalent metal ions such as aluminum and iron, and other ions such as tin, zirconium, titanium, and the like. The metal ion preferably includes a zinc ion. These cation components may be used alone or as a mixture of at least two of them.

The organic cation includes a cation having a carbon chain. The organic cation includes, for example, without limitation, an organic ammonium ion. Examples of the organic ammonium ion are: primary ammonium ions such as stearyl ammonium ion, hexyl ammonium ion, octyl ammonium ion, 2-ethyl hexyl ammonium ion, or the like; secondary ammonium ions such as dodecyl (lauryl) ammonium ion, octadecyl (stearyl) ammonium ion, or the like; tertiary ammonium ions such as trioctyl ammonium ion, or the like; and quaternary ammonium ion such as dioctyldimethyl ammonium ion, distearyldimethyl ammonium ion, or the like. These organic cations may be used alone or as a mixture of at least two of them.

Specific examples of (d) the mercaptobenzoic acids and/or the salts thereof include 4-mercaptobenzoic acid, 3-mercaptobenzoic acid, 2-mercaptobenzoic acid, chloromercaptobenzoic acid, bromomercaptobenzoic acid, methylmercaptobenzoic acid, ethylmercaptobenzoic acid, methoxymercaptobenzoic acid, ethoxymercaptobenzoic acid, hydroxymercaptobenzoic acid, cyanomercaptobenzoic acid, dimethylaminomercaptobenzoic acid, dimercaptobenzoic acid, and a salt thereof. Among them, mercaptobenzoic acid is preferred. (d) The mercaptobenzoic acids and/or the salts thereof may be used alone or as a mixture of at least two of them.

The content of at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof is preferably 1.5 parts by mass or more, more preferably 2.0 parts by mass or more, and is preferably 40 parts by mass or less, preferably 35 parts by mass or less, more preferably 30 parts by mass or less with respect to 100 parts by mass of (a) the base rubber. If the content is less than 1.5 parts by mass, the effect of adding at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof tends to be insufficient because of the low amount, and if the content is more than 40 parts by mass, the resilience of the core may be lowered, since the hardness of the resultant core may be lowered as a whole.

A mass ratio ((d)/(b)) of at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof to (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is preferably 0.01 or more, more preferably 0.02 or more, even more preferably 0.03 or more, further preferably 0.04 or more, and is preferably 0.5 or less, more preferably 0.4 or less, even more preferably 0.3 or less. If the mass ratio ((d)/(b)) is 0.01 or more, the effect of improving the flight distance by (d) the mercaptobenzoic acid and/or the salts thereof becomes large, and if the mass ratio ((d)/(b)) is 0.5 or less, lowering the hardness of the core is suppressed, and thus the rebound resilience becomes better.

In the case that the rubber composition used in the present invention contains only the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, the rubber composition further contains (e) a metal compound as an essential component. (e) The metal compound is not limited as long as it can neutralize (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubber composition. (e) The metal compound includes, for example, metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, copper hydroxide, and the like; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, copper oxide, and the like; metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, potassium carbonate, and the like. Among these, (e) the metal compound preferably includes a divalent metal compound, more preferably includes a zinc compound. The divalent metal compound reacts with the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, thereby forming a metal crosslinking. Use of the zinc compound provides a golf ball with excellent resilience. (e) These metal compounds are used solely or as a mixture of at least two of them. The content of (e) the metal compound may be appropriately determined in accordance with the desired degree of neutralization of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

The rubber composition used in the present invention preferably further contains (f) an organic sulfur compound. By using at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof and (f) the organic sulfur compound in combination for the rubber composition, the degree of the outer-hard and inner-soft structure of the core can be controlled to the higher extent. (f) The organic sulfur compound is a compound that is different from (d) the mercaptobenzoic acids and/or the salts thereof.

(f) The organic sulfur compound is not particularly limited, as long as it is an organic compound having a sulfur atom in the molecule thereof. Examples thereof include an organic compound having a thiol group (—SH), a polysulfide bond having 2 to 4 sulfur atoms (—S—S—, —S—S—S—, or —S—S—S—S—), or a metal salt thereof (—SM, —S-M-S—, —S-M-S—S—, —S—S-M-S—S—, —S-M-S—S—S—, or the like; M is a metal atom). Examples of the metal salts are salts of monovalent metals such as sodium, lithium, potassium, copper (I), and silver (I), and salts of divalent metals such as zinc, magnesium, calcium, strontium, barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel(II), zirconium(II), and tin (II). Furthermore, (f) the organic sulfur compound may be any one of aliphatic compounds (aliphatic thiol, aliphatic thiocarboxylic acid, aliphatic dithiocarboxylic acid, aliphatic polysulfides, or the like), heterocyclic compounds, alicyclic compounds (alicyclic thiol, alicyclic thiocarboxylic acid, alicyclic dithiocarboxylic acid, alicyclic polysulfides, or the like), and aromatic compounds.

(f) The organic sulfur compound includes, for example, thiophenols, thionaphthols, polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, and thiazoles. From the viewpoint that the hardness distribution of the spherical core becomes larger, (f) the organic sulfur compound preferably includes the organic sulfur compound having a thiol group (—SH) or the metal salt thereof, and more preferably includes thiophenols, thionaphthols or the metal salt thereof.

Examples of the thiols include, for example, thiophenols and thionaphthols. The thiophenols include thiophenol; thiophenols substituted with a fluoro group, such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol and the like; thiophenols substituted with a chloro group, such as 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, pentachlorothiophenol and the like; thiophenols substituted with a bromo group, such as 4-bromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, pentabromothiophenol and the like; thiophenols substituted with an iodo group, such as 4-iodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, pentaiodothiophenol and the like; or a metal salt thereof. As the metal salt, zinc salt is preferred.

Examples of the thionaphthols (naphthalenethiols) are 2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol, 2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and metal salts thereof. Preferable examples include 2-thionaphthol, 1-thionaphthol, or metal salts thereof. As the metal salts, preferred is salts of divalent metals, more preferred is a zinc salt. Specific examples of the metal salts are, for example, a zinc salt of 1-thionaphthol and zinc salt of 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond, and include, for example, disulfides, trisulfides, and tetrasulfides. The polysulfides preferably include diphenylpolysulfides.

Examples of the diphenylpolysulfides include diphenyldisulfide; diphenyldisulfides substituted with a halogen group, such as bis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide, bis(2,4,5,6-tetrafluorophenyl)disulfide, bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl)disulfide, bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide, bis(pentaiodophenyl)disulfide; diphenyldisulfides substituted with an alkyl group, such as bis(4-methylphenyl)disulfide, bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide, bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide, bis(penta-t-butylphenyl)disulfide; and the like.

The thiurams include, for example, thiurammonosulfides such as tetramethylthiuram monosulfide; thiuramdisulfides such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide; and thiuram tetrasulfides such as dipentamethylenethiuram tetrasulfide. The thiocarboxylic acids include a naphthalenethiocarboxylic acid. The dithiocarboxylic acids include, for example, a naphthalenedithiocarboxylic acid. The sulfenamides include, for example, N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide.

(f) The organic sulfur compound may be used solely or in combination of at least two of them. (f) The organic sulfur compound preferably includes thiophenols and/or the metal salt thereof, thionaphthols and/or the metal salt thereof, diphenyldisulfides and thiuramdisulfides, and more preferably includes 2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyldisulfide, bis(2,6-difluorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, and bis(pnetabromophenyl)disulfide.

The content of (f) the organic sulfur compound is preferably 0.05 part by mass or more, more preferably 0.1 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 2.0 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the content of (f) the organic sulfur compound is less than 0.05 part by mass, the effect of adding (f) the organic sulfur compound cannot be obtained and thus the resilience may not be enhanced. If the content of (f) the organic sulfur compound exceeds 5.0 parts by mass, the compression deformation amount of the obtained golf ball becomes large and thus the resilience may be lowered.

In the case of blending (f) the organic sulfur compound in the rubber composition, a mass ratio ((d)/(f)) of at least one compound of (d) the mercaptobenzoic acids and/or salts thereof to (f) the organic sulfur compound is preferably 2 or more, more preferably 3 or more, and is preferably 70 or less, more preferably 60 or less, even more preferably 50 or less.

The rubber composition used in the present invention may include additives such as a pigment, a filler for adjusting weight or the like, an antioxidant, a peptizing agent, and a softener where necessary.

Examples of the pigment blended in the rubber composition include a white pigment, a blue pigment, and a purple pigment. As the white pigment, titanium oxide is preferably used. The type of titanium oxide is not particularly limited, but rutile type is preferably used because of the high opacity. The content of titanium oxide is preferably 0.5 part by mass or more, and more preferably 2 parts by mass or more, and is preferably 8 parts by mass or less, and more preferably 5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber.

It is also preferred that the rubber composition contains both a white pigment and a blue pigment. The blue pigment is blended in order to cause white color to be vivid, and examples thereof include ultramarine blue, cobalt blue, and phthalocyanine blue. Examples of the purple pigment include anthraquinone violet, dioxazine violet, and methyl violet.

The content of the blue pigment is preferably 0.001 part by mass or more, and more preferably 0.05 part by mass or more, and is preferably 0.2 part by mass or less, and more preferably 0.1 part by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the content of the blue pigment is less than 0.001 part by mass, blueness is insufficient, and the color looks yellowish. If the content of the blue pigment exceeds 0.2 part by mass, blueness is excessively strong, and a vivid white appearance is not provided.

The filler blended in the rubber composition is used as a weight adjusting agent for mainly adjusting the weight of the golf ball obtained as a final product. The filler may be blended where necessary. The filler includes, for example, inorganic fillers such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, molybdenum powder, or the like. Zinc oxide is preferably used as the weight adjusting agent. It is considered that zinc oxide functions as a vulcanization activator and increases the hardness of the entire core. The content of the filler is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, even more preferably 20 parts by mass or less. If the content of the filler is less than 0.5 part by mass, it is difficult to adjust the weight, while if the content of the filler exceeds 30 parts by mass, the weight ratio of the rubber component is reduced and thus the resilience tends to be lowered.

The content of the antioxidant is preferably 0.1 part by mass or more and 1 part by mass or less, with respect to 100 parts by mass of (a) the base rubber. In addition, the content of the peptizing agent is preferably 0.1 part by mass or more and 5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber.

The rubber composition used in the present invention is obtained by mixing and kneading (a) the base rubber, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof, (c) the crosslinking initiator, and at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof, and other additives where necessary. The kneading can be conducted, without any limitation, with a well-known kneading machine such as a kneading roll, a banbury mixer, a kneader, or the like.

The spherical core of the golf ball of the present invention can be obtained by molding the rubber composition after kneaded. The temperature for molding the spherical core is preferably 120° C. or more, more preferably 150° C. or more, even more preferably 160° C. or more, and is preferably 170° C. or less. If the molding temperature exceeds 170° C., the surface hardness of the core tends to decrease. The molding pressure preferably ranges from 2.9 MPa to 11.8 MPa. The molding time preferably ranges from 10 minutes to 60 minutes.

The spherical core preferably has a hardness difference (Hs—Ho) between a surface hardness Hs and a center hardness Ho of 20 or more, more preferably 23 or more, even more preferably 25 or more, most preferably 27 or more, and preferably has a hardness difference of 80 or less, more preferably 70 or less, even more preferably 60 or less in JIS-C hardness. If the hardness difference between the center hardness and the surface hardness is large, the golf ball travelling a great flight distance due to the high launch angle and low spin rate is obtained.

The spherical core preferably has the center hardness Ho of 30 or more, more preferably 35 or more, even more preferably 40 or more in JIS-C hardness. If the center hardness Ho is less than 30 in JIS-C hardness, the core becomes so soft that the resilience may be lowered. Further, the spherical core preferably has the center hardness Ho of 70 or less, more preferably 65 or less, even more preferably 60 or less in JIS-C hardness. If the center hardness Ho exceeds 70 in JIS-C hardness, the core becomes too hard that the shot feeling tends to be lowered.

The spherical core preferably has the surface hardness Hs of 76 or more, more preferably 78 or more, and preferably has the surface hardness Hs of 100 or less, more preferably 95 or less in JIS-C hardness. If the surface hardness of the spherical core is 76 or more in JIS-C hardness, the spherical core does not become excessively soft, and thus the better resilience is obtained. Further, if the surface hardness of the spherical core is 100 or less in JIS-C hardness, the spherical core does not become excessively hard, and thus the better shot feeling is obtained.

The spherical core is preferably such that R² of a linear approximation curve obtained from a least square method is 0.85 or higher, when JIS-C hardness, which is measured at nine points obtained by dividing a radius of the spherical core into equal parts having 12.5% intervals therebetween, is plotted against distance (%) from a core center. The hardness of the spherical core is JIS-C hardness measured at nine points obtained by dividing a radius of the spherical core into equal parts having 12.5% interval. That is, JIS-C hardness is measured at nine points, namely at distances of 0% (core center), 12.5%, 25%, 37.5%, 50%, 62.5%, 75%, 87.5%, 100% (core surface) from the core center. Next, the measurement results are plotted to make a graph having JIS-C hardness as a vertical axis and distances (%) from the core center as a horizontal axis. In the present invention, R² of a linear approximation curve obtained from this graph by the least square method is preferably 0.85 or higher. R² of the linear approximation curve obtained by the least square method indicates the linearity of the obtained plot. In the present invention, R² of 0.85 or more means that the core has the hardness distribution where the hardness increases linearly or almost linearly. If the core having the hardness distribution where the hardness increases linearly or almost linearly is used for the golf ball, the spin rate on driver shots decrease. As a result, the flight distance on driver shots increases. R² of the linear approximation curve is preferably 0.90 or more, and more preferably 0.95 or more. The higher linearity provides a greater flight distance on driver shots.

The spherical core preferably has a ratio ((Hs—H₇₅)/(H₇₅—Ho)) of a hardness difference (Hs—H₇₅) between the surface hardness (Hs) and the 75% point hardness (H₇₅) thereof to a hardness difference (H₇₅—Ho) between the 75% point hardness (H₇₅) and the center hardness (Ho) thereof of 0.35 or less, more preferably has the ratio ((Hs—H₇₅)/(H₇₅—Ho)) of 0.33 or less, even more preferably has the ratio ((Hs—H₇₅)/(H₇₅—Ho)) of 0.30 or less. If the ratio ((Hs—H₇₅)/(H₇₅—Ho)) is 0.35 or less, the degree of the outer-hard inner-soft structure from the center to the 75% point becomes high, and the degree of the outer-hard inner-soft structure from the 75% point towards the surface becomes relatively small. That is, the spherical core has a relatively low hardness near the surface as well as a high degree of the outer-hard inner-soft structure as a whole. Thus, the resultant golf ball has a low spin rate on driver shots and a better shot feeling. The lower limit of the ratio ((Hs—H₇₅)/(H₇₅—Ho)) is not limited, but is preferably a positive number (more than 0).

The spherical core preferably has a diameter of 34.8 mm or more, more preferably 36.8 mm or more, and even more preferably 38.8 mm or more, and preferably has a diameter of 42.2 mm or less, more preferably 41.8 mm or less, and even more preferably 41.2 mm or less, and most preferably 40.8 mm or less. If the spherical core has a diameter of 34.8 mm or more, the thickness of the cover does not become too thick and thus the resilience becomes better. On the other hand, if the spherical core has a diameter of 42.2 mm or less, the thickness of the cover does not become too thin, and thus the cover functions better.

When the spherical core has a diameter from 34.8 mm to 42.2 mm, a compression deformation amount (shrinking deformation amount of the spherical core along the compression direction) of the spherical core when applying a load from 98 N as an initial load to 1275 N as a final load on the spherical core is preferably 2.0 mm or more, more preferably 2.8 mm or more, and is preferably 6.0 mm or less, more preferably 5.0 mm or less. If the compression deformation amount is 2.0 mm or more, the shot feeling of the golf ball becomes better. If the compression deformation amount is 6.0 mm or less, the resilience of the golf ball becomes better.

The golf ball cover of the present invention is formed from a cover composition containing a resin component. Examples of the resin component include, for example, an ionomer resin; a thermoplastic polyurethane elastomer having a commercial name of “Elastollan” commercially available from BASF Japan Ltd; a thermoplastic polyamide elastomer having a commercial name of “Pebax” commercially available from Arkema K. K.; a thermoplastic polyester elastomer having a commercial name of “Hytrel” commercially available from Du Pont-Toray Co., Ltd.; and a thermoplastic styrene elastomer having a commercial name of “Rabalon” commercially available from Mitsubishi Chemical Corporation; and the like.

The ionomer resin includes a product prepared by neutralizing at least a part of carboxyl groups in the binary copolymer composed of an olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with a metal ion, a product prepared by neutralizing at least a part of carboxyl groups in the ternary copolymer composed of an olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylic acid ester with a metal ion, or a mixture of those. The olefin preferably includes an olefin having 2 to 8 carbon atoms. Examples of the olefin are ethylene, propylene, butene, pentene, hexene, heptene, and octene. The olefin more preferably includes ethylene. Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms are acrylic acid, methacrylic acid, fumaric acid, maleic acid and crotonic acid. Among these, acrylic acid and methacrylic acid are particularly preferred. Examples of the α,β-unsaturated carboxylic acid ester include methyl ester, ethyl ester, propyl ester, n-butyl ester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid or the like. In particular, acrylic acid ester and methacrylic acid ester are preferable. Among these, the ionomer resin preferably includes the metal ion-neutralized product of the binary copolymer composed of ethylene-(meth)acrylic acid and the metal ion-neutralized product of the ternary copolymer composed of ethylene, (meth)acrylic acid, and (meth)acrylic acid ester.

Specific examples of the ionomer resins include trade name “Himilan (registered trademark) (e.g. the binary copolymerized ionomer such as Himilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706 (Zn), Himilan 1707 (Na), Himilan AM3711 (Mg); and the ternary copolymerized ionomer such as Himilan 1856 (Na), Himilan 1855 (Zn))” commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Further, examples include “Surlyn (registered trademark) (e.g. the binary copolymerized ionomer such as Surlyn 8945 (Na), Surlyn 9945 (Zn), Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn), Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li), Surlyn 7940 (Li), Surlyn AD8546 (Li); and the ternary copolymerized ionomer such as Surlyn 8120 (Na), Surlyn 8320 (Na), Surlyn 9320 (Zn), Surlyn 6320 (Mg), HPF 1000 (Mg), HPF 2000 (Mg))” commercially available from E.I. du Pont de Nemours and Company.

Further, examples include “lotek (registered trademark) (e.g. the binary copolymerized ionomer such as lotek 8000 (Na), lotek 8030 (Na), lotek 7010 (Zn), lotek 7030 (Zn); and the ternary copolymerized ionomer such as lotek 7510 (Zn), lotek 7520 (Zn))” commercially available from ExxonMobil Chemical Corporation.

It is noted that Na, Zn, Li, and Mg described in the parentheses after the trade names indicate metal types of neutralizing metal ions for the ionomer resins.

The cover composition constituting the cover of the golf ball of the present invention preferably includes, as a resin component, a thermoplastic polyurethane elastomer or an ionomer resin. In case of using the ionomer resin, it is preferred to use a thermoplastic styrene elastomer together. The content of the polyurethane or ionomer resin in the resin component of the cover composition is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more.

In the present invention, the cover composition may further contain a pigment component such as a white pigment (for example, titanium oxide), a blue pigment, and a red pigment; a weight adjusting agent such as zinc oxide, calcium carbonate, and barium sulfate; a dispersant; an antioxidant; an ultraviolet absorber; a light stabilizer; a fluorescent material or a fluorescent brightener; and the like, as long as they do not impair the effect of the present invention.

The content of the white pigment (for example, titanium oxide) is preferably 0.5 part or more, more preferably 1 part or more, and the content of the white pigment is preferably 10 parts or less, more preferably 8 parts or less, with respect to 100 parts of the resin component constituting the cover by mass. If the content of the white pigment is 0.5 part by mass or more, it is possible to impart the opacity to the resultant cover. Further, if the content of the white pigment is more than 10 parts by mass, the durability of the resultant cover may deteriorate.

The slab hardness of the cover composition is preferably set in accordance with the desired performance of the golf balls. For example, in case of a so-called distance golf ball which focuses on a flight distance, the cover composition preferably has a slab hardness of 50 or more, more preferably 55 or more, and preferably has a slab hardness of 80 or less, more preferably 70 or less in shore D hardness. If the cover composition has a slab hardness of 50 or more, the obtained golf ball has a high launch angle and low spin rate on driver shots and iron shots, and thus the flight distance becomes large. If the cover composition has a slab hardness of 80 or less, the golf ball excellent in durability is obtained.

Further, in case of a so-called spin golf ball which focuses on controllability, the cover composition preferably has a slab hardness of less than 50, and preferably has a slab hardness of 20 or more, more preferably 25 or more in shore D hardness. If the cover composition has a slab hardness of less than 50, the flight distance on driver shots can be improved by the core of the present invention, as well as the obtained golf ball readily stops on the green due to the high spin rate on approach shots. If the cover composition has a slab hardness of 20 or more, the abrasion resistance improves. In case of a plurality of cover layers, the slab hardness of the cover composition constituting each layer can be identical or different, as long as the slab hardness of each layer is within the above range.

An embodiment for molding a cover is not particularly limited, and includes an embodiment which comprises injection molding the cover composition directly onto the core, or an embodiment which comprises molding the cover composition into a hollow-shell, covering the core with a plurality of the hollow-shells and subjecting the core with a plurality of the hollow shells to the compression-molding (preferably an embodiment which comprises molding the cover composition into a half hollow-shell, covering the core with the two half hollow-shells, and subjecting the core with the two half hollow-shells to the compression-molding).

When molding the cover in a compression molding method, molding of the half shell can be performed by either compression molding method or injection molding method, and the compression molding method is preferred. The compression-molding of the cover composition into half shell can be carried out, for example, under a pressure of 1 MPa or more and 20 MPa or less at a temperature of −20° C. or more and 70° C. or less relative to the flow beginning temperature of the cover composition. By performing the molding under the above conditions, a half shell having a uniform thickness can be formed. Examples of a method for molding the cover using half shells include compression molding by covering the core with two half shells. The compression molding of half shells into the cover can be carried out, for example, under a pressure of 0.5 MPa or more and 25 MPa or less at a temperature of −20° C. or more and 70° C. or less relative to the flow beginning temperature of the cover composition. By performing the molding under the above conditions, a golf ball cover having a uniform thickness can be formed.

In the case of directly injection molding the cover composition, the cover composition extruded in the pellet form beforehand may be used for injection molding or the materials such as the base resin components and the pigment may be dry blended, followed by directly injection molding the blended material. It is preferred to use upper and lower molds having a spherical cavity and pimples for forming a cover, wherein a part of the pimples also serves as a retractable hold pin. When molding the cover by injection molding, the hold pin is protruded to hold the core, and the cover composition which has been heated and melted is charged and then cooled to obtain a cover. For example, it is preferred that the cover composition heated and melted at the temperature ranging from 200° C. to 250° C. is charged into a mold held under the pressure of 9 MPa to 15 MPa for 0.5 to 5 seconds, and after cooling for 10 to 60 seconds, the mold is opened and the golf ball with the cover molded is ejected from the mold.

The concave portions called “dimple” are usually formed on the surface of the cover. The total number of the dimples is preferably 200 or more and 500 or less. If the total number is less than 200, the dimple effect is hardly obtained. On the other hand, if the total number exceeds 500, the dimple effect is hardly obtained because the size of the respective dimples is small. The shape (shape in a plan view) of dimples includes, for example, without limitation, a circle, polygonal shapes such as roughly triangular shape, roughly quadrangular shape, roughly pentagonal shape, roughly hexagonal shape, and another irregular shape. The shape of the dimples is employed solely or at least two of them may be used in combination.

In the present invention, the thickness of the cover of the golf ball is preferably 4.0 mm or less, more preferably 3.0 mm or less, even more preferably 2.0 mm or less. If the thickness of the cover is 4.0 mm or less, the resilience and shot feeling of the obtained golf ball become better. The thickness of the cover is preferably 0.3 mm or more, more preferably 0.5 mm or more, and even more preferably 0.8 mm or more, and most preferably 1.0 mm or more. If the thickness of the cover is less than 0.3 mm, the durability and the wear resistance of the cover may deteriorate. If the cover has a plurality of layers, it is preferred that the total thickness of the cover layers falls within the above range.

After the cover is molded, the mold is opened and the golf ball body is ejected from the mold, and as necessary, the golf ball body is preferably subjected to surface treatments such as deburring, cleaning, and sandblast. If desired, a paint film or a mark may be formed. The paint film preferably has a thickness of, but not limited to, 5 μm or larger, and more preferably 7 μm or larger, and preferably has a thickness of 50 μm or smaller, and more preferably 40 μm or smaller, even more preferably 30 μm or smaller. If the thickness is smaller than 5 μm, the paint film is easy to wear off due to continued use of the golf ball, and if the thickness is larger than 50 μm, the effect of the dimples is reduced, resulting in lowering flying performance of the golf ball.

When the golf ball of the present invention has a diameter in a range from 40 mm to 45 mm, a compression deformation amount of the golf ball (shrinking amount of the golf ball in the compression direction thereof) when applying a load from an initial load of 98 N to a final load of 1275 N to the golf ball is preferably 2.0 mm or more, more preferably 2.4 mm or more, even more preferably 2.5 mm or more, most preferably 2.8 mm or more, and is preferably 5.0 mm or less, more preferably 4.5 mm or less. If the compression deformation amount is 2.0 mm or more, the golf ball does not become excessively hard, and thus exhibits a good shot feeling. On the other hand, if the compression deformation amount is 5.0 mm or less, the resilience is enhanced.

The golf ball construction is not limited, as long as the golf ball of the present invention comprises a spherical core and at least one cover layer covering the spherical core. FIG. 1 is a partially cutaway sectional view showing the golf ball 2 according to the preferable embodiment of the present invention. The golf ball 2 comprises a spherical core 4, and a cover 12 covering the spherical core 4. Plurality of dimples 14 are formed on a surface of the cover. Other portions than dimples 14 on the surface of the golf ball 2 are land 16. The golf ball 2 is provided with a paint layer and a mark layer outside the cover 12, but these layers are not depicted.

The spherical core preferably has a single layered structure. Unlike the multi-layered structure, the spherical core of the single layered structure does not have an energy loss at the interface of the multi-layered structure when hitting, and thus has an improved resilience. The cover has a structure of at least one layer, for example a single layered structure, or a multi-layered structure of at least two layers. The golf ball of the present invention includes, for example, a two-piece golf ball comprising a spherical core and a single layered cover disposed around the spherical core; a multi-piece golf ball comprising a spherical core, and at least two cover layers disposed around the spherical core (including the three-piece golf ball); and a wound golf ball comprising a spherical core, a rubber thread layer which is formed around the spherical core, and a cover disposed over the rubber thread layer. The present invention can be suitably applied to any one of the above golf balls.

EXAMPLES

Hereinafter, the present invention will be described in detail by way of example. The present invention is not limited to examples described below. Various changes and modifications can be made without departing from the spirit and scope of the present invention.

[Evaluation Methods] (1) Compression Deformation Amount (mm)

The compression deformation amount of the core or golf ball (shrinking deformation amount of the core or golf ball in the compression direction thereof), when applying a load from 98 N as an initial load to 1275 N as a final load to the core or golf ball, was measured.

(2) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection molding the cover composition, and stored at 23° C. for two weeks. Three or more of these sheets were stacked on one another so as not to be affected by the measuring substrate on which the sheets were placed, and the hardness of the stack was measured with a type P1 auto loading durometer manufactured by Kobunshi Keiki Co., Ltd., provided with a Shore D type spring hardness tester prescribed in ASTM-D2240.

(3) Hardness Distribution of Spherical Core (JIS-C Hardness)

A type P1 auto loading durometer manufactured by Kobunshi Keiki Co., Ltd., provided with a JIS-C type spring hardness tester was used to measure the hardness of the spherical core. The hardness measured at the surface of the spherical core was adopted as the surface hardness of the spherical core. The spherical core was cut into two hemispheres to obtain a cut plane, and the hardness were measured at the central point and at predetermined distances from the central point. The core hardness were measured at 4 points at predetermined distances from the central point of the cut plane of the core. The core hardness was calculated by averaging the hardness measured at 4 points.

(4) Spin Rate (rpm) on Driver Shots

A metal-headed W#1 driver (XXIO, Shaft: S, loft: 11°, manufactured by Dunlop Sports Limited) was installed on a swing robot M/C manufactured by Golf Laboratories, Inc. A golf ball was hit at a head speed of 40 m/sec, and the spin rate right after hitting the golf ball was measured. This measurement was conducted twelve times for each golf ball, and the average value was adopted as the measurement value for the golf ball. A sequence of photographs of the hit golf ball were taken for measuring the spin rate (rpm) right after hitting the golf ball. The spin rate on driver shots of golf balls is shown as the difference from that of the golf ball (core) No. 9.

(5) Shot Feeling

An actual hitting test was carried out by ten amateur golfers (high skilled person) using a sand wedge (CG15 forged wedge)(52° available from Cleveland Golf). In accordance with the number who answered the shot feeling was good (feeling like that the golf ball was lifted on the club face, feeling like that the golf ball gripped on the club face, feeling like that the spin is imparted, feeling like that the golf ball was stuck on the club face, etc.), the golf balls were evaluated as follows.

E (Excellent): 8 or more

G (Good): 4 to 7

P (Poor): 3 or less

[Production of Golf Balls] (1) Production of Cores

The rubber compositions having formulations shown in Table 3 were kneaded with a kneading roll and heat-pressed in upper and lower molds, each having a hemispherical cavity, at 170° C. for 20 minutes to prepare spherical cores having a diameter of 39.8 mm.

TABLE 3 Golf ball No. 1 2 3 4 5 6 7 8 9 10 Rubber (a) BR730 100 100 100 100 100 100 100 100 100 100 composition (b) Sanceler SR 35 31.5 29 25 27 37 32 29 23 27 (part by (e) Zinc oxide 5 5 5 5 5 5 5 5 5 5 mass) Barium sulfate *1) *1) *1) *1) *1) *1) *1) *1) *1) *1) (f) 2-Thionaphthol 0.2 0.2 0.2 0.1 0.1 0.2 0.1 0.1 — 0.2 (d) 4-Mercaptobenzoic acid 2 4 6.3 — — — — — — — Benzoic acid — — — 0.74 2.3 5.2 — — — — 2,4,5-Trichlorothiophenol — — — — — — 3 6.1 — — (c) Dicumyl peroxide 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Core Center hardness (Ho) 48.6 42.9 39.0 54.0 47.7 45.7 49.6 50.1 56.9 54.5 hardness 12.5% point hardness 59.1 51.9 47.0 64.5 59.2 49.7 57.9 56.2 62.2 61.7 distribution 25% point hardness 66.0 58.9 55.3 69.3 66.4 55.5 64.5 62.6 65.4 65.4 (JIS-C) 37.5% point hardness 68.0 62.4 60.6 70.7 69.1 59.5 66.7 64.6 66.7 66.6 50% point hardness 68.8 64.8 64.5 71.1 69.4 61.5 67.3 65.7 66.8 66.5 62.5% point hardness 68.6 68.7 70.4 70.0 68.5 61.5 70.1 68.3 67.5 65.7 75% point hardness (H₇₅) 74.4 75.7 76.0 73.6 76.0 74.6 73.1 71.2 72.1 72.0 87.5% point hardness 77.6 78.3 77.9 78.3 80.2 83.0 71.8 70.1 73.4 75.4 Surface hardness (Hs) 83.5 84.2 81.9 85.0 86.4 88.9 79.3 77.0 80.1 82.1 Surface hardness − center hardness 34.9 41.3 42.9 31.0 38.7 43.2 29.7 26.9 23.2 27.6 (Hs-H₇₅)/(H₇₅-Ho) 0.35 0.26 0.16 0.58 0.37 0.49 0.26 0.27 0.53 0.58 R² of approximated curve 0.90 0.98 0.98 0.85 0.90 0.94 0.89 0.92 0.92 0.88 Slope of approximated curve 0.28 0.38 0.42 0.23 0.32 0.42 0.24 0.23 0.19 0.22 Core compression deformation amount (mm) 4.09 4.10 4.10 4.06 4.02 4.10 4.15 4.24 4.16 4.19 Cover composition A A A A A A A A A A Slab hardness (Shore D) 65 65 65 65 65 65 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −50 −100 −100 15 −60 −120 −20 −10 0 −10 Compression deformation amount (mm) 3.39 3.40 3.40 3.36 3.32 3.40 3.45 3.54 3.46 3.49 Shot feeling E E E G P P E E E E *1) As to an amount of barium sulfate, adjustment was made such that the golf ball had a mass of 45.4 g. BR730: a high-cis polybutadiene (cis-1,4 bond content = 96 mass %, 1,2-vinyl bond content = 1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.) = 55, molecular weight distribution (Mw/Mn) = 3) available from JSR Corporation Sanceler SR: zinc acrylate (product of 10 mass % stearic acid coating) available from Sanshin Chemical Industry Co., Ltd. Zinc oxide: “Ginrei R” manufactured by Toho Zinc Co., Ltd. Barium sulfate: “Barium sulfate BD” manufactured by Sakai Chemical Industry Co., Ltd., adjustment was made such that the finally obtained golf ball had a mass of 45.4 g. 2-Thionaphthol: available from Tokyo Chemical Industry Co., Ltd. 4-Mercaptobenzoic acid: available from Tokyo Chemical Industry Co., Ltd. Benzoic acid: available from Sigma-Aldrich Co., Ltd. 2,4,5-Trichlorothiophenol: available from Wako Pure Chemical Industries, Ltd. Dicumyl peroxide: “PERCUMYL (trade mark) D” available from NOF Corporation.

(2) Production of Cover

Next, cover materials shown in Table 4 were extruded with a twin-screw kneading extruder to prepare the cover composition in the pellet form. The extruding conditions of the cover composition were a screw diameter of 45 mm, a screw rotational speed of 200 rpm, and screw L/D=35, and the mixtures were heated to 150 to 230° C. at the die position of the extruder. The obtained cover compositions were injection-molded onto the spherical cores obtained above to produce the golf balls having the spherical core and the cover covering the spherical core.

TABLE 4 Cover composition No. A Himilan 1605 50 Himilan 1706 50 Titanium oxide 4 Slab hardness (Shore D) 65 Formulation: parts by mass Himilan 1605: Sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd Himilan 1706: Zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd

Apparent from the result in Table 3, the golf ball Nos. 1 to 3 using at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof have a high degree of the outer-hard inner-soft structure, and have a spin rate on driver shots lower than that of the golf ball Nos. 9, 10 and the golf ball Nos. 7, 8 (2,4,5-trichlorothiophenol was blended therein). Further, when comparing the cores having a compression deformation amount at the same level, it is found that the golf ball Nos. 1 to 3 using at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof have a spin rate on driver shots at the same level as that of the golf ball No. 6 (benzoic acid was blended therein), but the shot feeling thereof is improved.

The golf ball of the present invention has a low spin rate on driver shots and an excellent shot feeling. This application is based on Japanese Patent application No. 2013-181043 filed on Sep. 2, 2013, the content of which are hereby incorporated by reference. 

1. A golf ball having a spherical core and at least one cover covering the spherical core, wherein the spherical core is formed from a rubber composition containing: (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and at least one compound of (d) mercaptobenzoic acids and/or salts thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, and the spherical core has a hardness difference (Hs—Ho) between a surface hardness (Hs) and a center hardness (Ho) thereof of 20 or more in JIS-C hardness.
 2. The golf ball according to claim 1, wherein the rubber composition contains at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof in an amount ranging from 1.5 parts by mass to 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.
 3. The golf ball according to claim 1, wherein the rubber composition further contains (f) an organic sulfur compound different from the at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof.
 4. The golf ball according to claim 3, wherein (f) the organic sulfur compound includes at least one compound selected from the group consisting of thiophenols and/or metal salts thereof, thionaphthols and/or metal salts thereof, diphenylpolysulfides and thiuramdisulfides.
 5. The golf ball according to claim 3, wherein the rubber composition contains (f) the organic sulfur compound in an amount ranging from 0.05 part by mass to 5 parts by mass with respect to 100 parts by mass of (a) the base rubber.
 6. The golf ball according to claim 3, wherein the rubber composition contains mercaptobenzoic acid as at least one compound of (d) the mercaptobenzoic acids and/or the salts thereof, and 2-thionaphthol and/or 1-thionaphthol as (f) the organic sulfur compound.
 7. The golf ball according to claim 1, wherein the hardness difference (Hs—Ho) is 80 or less in JIS-C hardness.
 8. The golf ball according to claim 1, wherein the center hardness (Ho) is 30 or more and 70 or less in JIS-C hardness.
 9. The golf ball according to claim 1, wherein the surface hardness (Hs) is 76 or more and 100 or less in JIS-C hardness.
 10. The golf ball according to claim 1, wherein the spherical core has a ratio ((Hs—H₇₅)/(H₇₅—Ho)) of a hardness difference (Hs—H₇₅) between the surface hardness (Hs) and a 75% point hardness (H₇₅) thereof to a hardness difference (H₇₅—Ho) between the 75% point hardness (H₇₅) and the center hardness (Ho) thereof of 0.35 or less.
 11. The golf ball according to claim 1, wherein the spherical core is such that R² of a linear approximation curve obtained from a least square method is 0.85 or higher, when JIS-C hardness, which is measured at nine points obtained by dividing a radius of the spherical core into equal parts having 12.5% intervals therebetween, is plotted against distance (%) from a core center. 